Method and apparatus for media access in contention-based networks

ABSTRACT

A method and apparatus are described for gaining access to a communication medium in a contention-based network, including determining a slot count based on a number of stations in the contention-based network, adjusting the slot count, initiating a frame transmission when the slot count reaches a predetermined value and wherein said number of stations and an address queue are adjusted to reflect a priority. Further, a method and apparatus are described for gaining access to a communication medium in a contention-based network, including receiving a slot count based on a number of stations in the contention-based network, adjusting the slot count, initiating a frame transmission when the slot count reaches a predetermined value and wherein said number of stations and an address queue are adjusted to reflect a priority.

FIELD OF THE INVENTION

The present invention relates to media access in contention-basednetworks and in particular gaining access to a communication medium incontention-based networks by reduction or elimination of contention incontention-based networks.

BACKGROUND OF THE INVENTION

The media access control (MAC) layer's primary function is to provide afair mechanism to control access of shared communication media. However,in a wireless communication media such as IEEE 802.11 WLAN, prior totransmitting a frame, the MAC layer must gain access to the network,which it does through two different access mechanisms: acontention-based mechanism, called the distributed coordination function(DCF) and a centrally controlled access mechanism, called the pointcoordination function (PCF).

The PCF modes allow the implementation of a quality of service (QoS)mechanism, but it is optional and requires extra interactions in orderto negotiate a QoS between the mobile terminal and the access point(AP). The DCF mode, considered as the default mode, does not provide anyQoS mechanism. Consequently all stations including the base station APin a wireless local area network (WLAN) have the same probability ofacquiring access to the medium and sending data via the medium. Thistype of service is referred to as a “best effort”.

Three inter-frame space (IFS) intervals defer an IEEE 802.11 station'saccess to the medium and provide various levels of priority. Eachinterval defines the duration between the end of the last symbol of theprevious frame to the beginning of the first symbol of the next frame.The Short Interframe Space (SIFS) provides the highest priority level byallowing some frames to access the medium before others, such as an ACKframe, a Clear-to-Send (CTS) frame, or a fragment of a previous dataframe.

Simultaneous transmit attempts from a number of wireless stations leadto collisions in both the downlink and the uplink communication media,since only one transport stream can be transmitted during any oneperiod. The problem is particularly acute during periods of high trafficloads and may render the protocol unstable. The IEEE 802.11 MAC layeruses collision avoidance rather than collision detection in order tosimultaneously transmit and receive data. To resolve collisions,subsequent transmission attempts are typically staggered randomly intime using a binary exponential backoff. The DCF uses physical andvirtual carrier sense mechanisms (carrier sense multiple access withcollision avoidance (CSMA/CA)) with a binary exponential backoff thatallows access attempts after sensing the channel for activity.

The backoff procedure for the family of IEEE 802.11 standards was firstintroduced for the DCF mode as the basic solution for collisionavoidance, and further employed by the IEEE 802.11e to solve the problemof internal collisions between enhanced distributed channel accessfunctions (EDCAFs). In the emerging IEEE 802.11n standard, the backoffprocedure is still used as the fundamental approach for supportingdistributed access among mobile stations. Currently, almost allcommercially available wireless products of the IEEE 802.11 series useDCF/EDCAF as the solution for medium access and thus heavily depend onthe backoff mechanism to avoid collisions. As used herein, “/” denotesalternative names for the same or similar components or structures. Thatis, a “/” can be taken as meaning “or” as used herein.

The principle and operations of the exponential random backoff procedureare similar in both standards. In order to set the background for thepresent invention, the backoff procedure specified in IEEE 802.11 isdescribed. Before transmitting each frame, a mobile station (includingaccess point (AP)) determines the state of the wireless medium byphysical or Virtual carrier sensing, and if busy, the station chooses arandom integer uniformly distributed between 0 and the contention window(CW) as the initial value of the slot count for backing off. Once themedium is determined to be idle after a DCF inter-frame space (DIFS)plus the random number of slot count, where the mobile station decreasesthe slot count by one for each slot time, then the mobile station cantransmit. This procedure is suspended if the medium is determined to bebusy at any time during backing off. The contention window (CW)increases exponentially upon each unsuccessful transmission attempt. Itbegins with a minimum value CWmin and increases up to a maximum valueCWmax. All parameters related to the backoff procedure, including theslot time, DIPS, CWmin and CWmax, are specified for the physical layer.

FIG. 1 is an exemplary representation of the random backoff proceduredescribed above. A wireless local area network (WLAN) with one accesspoint and three associated mobile stations is considered in thisscenario. As used herein, an access point includes bridges, routers andbrouters and any other device used by stations to access a network. AnAP also acts as an interconnection point between a radio network(wireless network) and a wired local area network (LAN). In FIG. 1, tworounds of medium contention are shown. To start, the access point (AP)transmits a frame. When the transmission concludes, the medium becomesidle. After the medium is determined to be idle without interruption fora period of time equal to DIFS, all stations including the AP start theexponential random backoff procedure to contend for the medium. At thismoment each station maintains a slot count for backing off. For the APthat wins the contention in the previous round of contention, its slotcount is randomly chosen from contention window [0, CW], while otherstations retain their slot count as in the previous round. The slotcount is used to determine how long the station has to wait to determineif the medium is busy before it can transmit. As shown in FIG. 1, duringthe first round of contention, the random number used for the slot countfor the AP is 7. The slot count for station 1 is 8. For station 2 theslot count is 5 and for station 3 the slot count is 3. As each time slotelapses and the medium remains idle, all stations decrease their slotcount by one respectively. Since station 3 has the smallest backoff slotcount (3), station 3 wins the contention after the medium is idle for aperiod of 3 time slots and station 3 initiates a new frame transmissionat the 4^(th) time slot. Note that as of the time station 3 transmits,other stations have decreased their slot count by three. When station 3completes its transmission, the second round of contention begins andstation 3 randomly chooses a value 8 from contention window [0, CW] asits slot count. As in the first round, other stations use theirremaining slot count for backing off. Now, the AP has a slot count of 4.Station 1 has a slot count of 5. Station 2 has a slot count of 2 andstation 3 has a slot count of 8. In this round of contention, station 2has the shortest slot count so it wins and transmits a frame after theDIFS period plus two slot times. This procedure repeats throughout thelifetime of the network.

A major deficiency of the random backoff procedure lies in that, therandomly chosen value of the slot count may degenerate the utility ofthe medium and thus degrade the performance of carrier sense multipleaccess (CSMA) technique. Two factors may cause the degeneration. First,as specified in the standard, the station with the smallest backoff time(slot count) is the winner to access the medium, thus a period when themedium is idle exists before next transmission. The existence of such avacancy between successive frame transactions negatively influences theefficiency of the backoff procedure. The second factor is thepossibility of collisions among multiple stations. Though it is greatlyrelieved by the adoption of randomization during selection of thebackoff slots, its negative impact on the network performance can stillnot be neglected, particularly when the number of contending stations islarge.

Another deficiency of the random backoff procedure is the lack offairness among the stations. The method that doubles the contentionwindow upon unsuccessful transmission may put a station at adisadvantage during the next contention interval/period, as it inclinesto choose a slot count larger than its counterparts. Such a binary,exponentially doubled backoff procedure severely defers access to themedium, and may lead to the bandwidth starvation in some cases.Experience has shown that the difference of throughput of stationswithin the same network may reach 30% of the average.

Many backoff schemes have been proposed to overcome these issues. In oneprior art backoff scheme, a multiplicative increase and linear decrease(MILD) algorithm was introduced to change the backoff contention windowin a moderate way, and thus improve the fairness. In another prior atscheme, to achieve increased fairness among stations, the contentionwindow is changed dynamically with the estimated fair share of channelassigned to each station. In yet another prior art scheme, a generalmechanism is presented for translating a given-fairness model into acorresponding contention resolution scheme. A backoff algorithm thatachieves proportional fairness is derived using the contentionresolution scheme. In yet another prior at scheme, the probabilitydistribution of slot selection is considered, and an exponential randomwalking backoff algorithm is proposed in which the backoff slot countdecrements with a predetermined probability. In yet another media accessscheme, time slots are assigned to each station, where the number oftime slots is at least as great as the number of stations in thenetwork. This scheme essentially replaces a contention-based frequencydivision scheme, such as CSMA, by a time division multiple access schemewhere time slots are assigned to each station. A great deal of researchhas been done in the area of backoff algorithms, fairness and quality ofservice remain largely unresolved.

Thus, it would be advantageous to have a solution to the fairness andquality of service issues of the random backoff procedures specified bythe IEEE 802.11 standards.

SUMMARY OF THE INVENTION

Described herein is a new backoff method that seeks to improve theperformance of the legacy random backoff procedure. The method of thepresent invention adopts a different approach to resolve externalcollisions. Deterministic values are selected for the backoff slotcounts. Thus, there is no duplication among distributed slot counts andeach station can exclusively access the medium without colliding withothers. By cycling the slot count though a fixed interval [0, N], whereN is the number of stations in the network, the method of the presentinvention offers a round robin type service among the stations.Therefore, the method of the present invention provides guaranteedfairness for the network, and furthermore analysis has shown that themethod has high network efficiency for moderate to heavy traffic loads.The round robin type service cycling through the N stations amounts toscheduling stations such that each station receives a fair amount oftime. The stations of the present invention can be mobile or fixed andthe network can be wired line or wireless. The present invention isdirected to any contention-based network where the station uses aphysical or virtual carrier sense mechanism to determine if the networkis busy. This would include any networks whose MAC layer protocol buildson CSMA, such as cable networks. However, the station may also bestationary and the network can be any contention-based network.

A major feature of the method of the present invention is that, undersaturated traffic scenarios, the time interval/period between successiveframe exchange sequences initiated by two separate stations is only oneDIFS plus one slot time. Such an inter-space time interval/period isshorter than that of conventional random backoff methods used byDCF/EDCA (Enhanced Distributed Channel Access), but longer than thePoint Coordination Function (PCF) time interval/period that is used bythe PCF/HCCA (HCF Controlled Channel Access) mechanism. Moreover, themethod of the present invention regulates a sequential service orderamong the stations, while conventional random backoff methods do nothave such a feature.

Network collisions are an annoying issue for the CSMA based wirelesscommunications, as collisions greatly degenerate network performance,particularly in terms of throughput and network efficiency. However,collisions are eliminated (or greatly reduced) in the deterministicbackoff (communication medium access) method of the present invention.Each station can exclusively take control of the wireless medium afterits slot count reaches zero. In this sense, the deterministic backoffmethod of the present invention outperforms legacy random backoffmethods.

A method and apparatus are described for gaining access to acommunication medium in a contention-based network, includingdetermining a slot count based on a number of stations in thecontention-based network, adjusting the slot count, initiating a frametransmission when the slot count reaches a predetermined value andwherein said number of stations and an address queue are adjusted toreflect a priority. Further, a method and apparatus are described forgaining access to a communication medium in a contention-based network,including receiving a slot count based on a number of stations in thecontention-based network; adjusting the slot count, initiating a frametransmission when the slot count reaches a predetermined value andwherein said number of stations and an address queue are adjusted toreflect a priority.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Thedrawings include the following figures briefly described below wherelike-numbers on the figures represent similar elements:

FIG. 1 is an exemplary illustration of the results of applying theconventional random backoff procedure employed by DCF and EDCAF.

FIG. 2 is an exemplary illustration of the results of applying thedeterministic backoff mechanism of the present invention.

FIG. 3 illustrates an embodiment of the format of the management framein accordance with the principles of the present invention.

FIG. 4 a is a flowchart of the operation of the deterministic backoffmethod of the present invention from the perspective of the accesspoint.

FIG. 4 b is a flowchart of the operation of the deterministic backoffmethod of the present invention from the perspective of the station.

FIG. 5 a is a schematic/block diagram of the operation of an AP ingaining access to a communication medium in a contention-based networkin accordance with the principles of the present invention.

FIG. 5 b is a schematic/block diagram of the operation of a station ingaining access to a communication medium in a contention-based networkin accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The prior art random backoff mechanism, including its subsequentvariants, relies on the randomness when choosing the initial backoffslot count (shortened as initial count below) during each round ofcontention. As each station chooses its initial counter independently,the probability for any two or more of the station to have an identicalslot count concurrently is low. The actual value may depend on thenumber of contending stations, the contention window (CW) used and thedistribution function of the initial count. In most cases, the CW ismuch larger than the number of contending stations, thus low probabilityof potential collision can be expected for each attempted transmission.The random backoff mechanism is based on this low probability ofcollision.

The present invention resolves the issue of network collision using anentirely different method. Observing that the main cause of collisionsis the overlapped slot count used by multiple stations, the method ofthe present invention selects the initial count in a more predictable ordeterministic way. The method of the present invention uses uniqueinformation for each station to derive its initial count, and the methodguarantees that at any slot time, each station holds a slot count thatis unique for the network. Thus, exclusive medium access can be achievedfor each attempt. As the method of the present invention selects theinitial count in a deterministic way, it is denominated hereindeterministic backoff.

In the deterministic backoff method of the present invention, eachstation decrements its slot, count whenever the medium has been sensedto be idle for a DIFS time. When the slot count reaches zero, a stationstarts transmitting, for example, in a wireless network over the air.These operations are similar to that of the conventional random backoffmechanism. Discrepancies appear when a mobile station needs to choose aninitial count for backing off upon conclusion of a frame exchange or atransmission failure. For the deterministic backoff method of thepresent invention, the initial count is chosen based on the globalunique information of the network—the number of stations and to avoidoverlapping with the slot counts of other stations. The method used bythe conventional random backoff that doubles the size of the contentionwindow upon collision or failure is replaced by the round robin serviceof the present invention, in which the slot count cycles through a fixedinterval [0, N], with N being an integer equal to the number of stationsin the network (including the AP). The round robin service of thepresent invention operates throughout the entire service period. Oncethe slot count reaches zero, a station gets an opportunity to initiate aframe exchange sequence. Afterwards, the slot count restarts with avalue of N. The slot count should be set to an initial value during thestation's first attempt to access the medium and adjusted adaptively asthe network evolves. Besides, considering that there is a possibility oftemporal non-synchronization of slot time among multiple stations andthe clear channel assessment (CCA) error, a slot count calibrationprocedure is adopted in the present invention.

FIG. 2 is an exemplary illustration of the deterministic backoffmechanism of the present invention. A wireless LAN with one AP and threemobile stations is considered in this example. As shown in the figure,each station maintains a slot count that cycles through 0 to 4, andwhenever it reaches 0, a frame transmission commences, followed byresetting the slot count to 4. For each round of backing off, there isalways one station that will be served after a period of a DIFS and oneslot time. Thus like the token ring networks, the opportunity of gainingcontrol of the wireless medium circulates among these stations, in apredefined sequence. As for the example in FIG. 2, the serving order isAP→STA 1→STA 2→STA 3→AP, and the intermediate time space betweensuccessive serving opportunities is a DIFS plus one slot time, asubstantially shorter time compared to conventional random backoffmethods.

Referring still to FIG. 2, after the AP transmitted a frame, it resetsits slot count to four, which is the maximum number of stations (N) inthis case. When the next round of contention begins, all stationsincluding the AP decrease their slot counts by one after the medium issensed to be idle for a DIPS time. Because STA 1 has the smallest slotcount (one), it wins the contention and gets the opportunity to transmita frame after a slot time elapses and slot count reaches zero. Otherstations suspend decreasing their slot count during the transmission bySTA 1. Once STA 1 concludes its transmission, it resets its slot countto four and a new round of backoff begins. This continues in this mannerfor the life of the network with each station getting a turn to transmitin order. Should a new station wish to associate with the network, theslot count is increased by one by the AP and distributed by the AP toeach currently associated station and to the new station in a field ofthe management frame or piggybacked onto the association response. Notethat the slot count distribution procedure should be conductedimmediately in a SIFS period after the conclusion of the associationprocedure to prevent other stations decrementing their slot count in themiddle of the two procedures.

In the view of an individual station, the entire procedure ofdeterministic backoff can be described using two parameters: the initialslot count for a station to access the network for the first time as anew member of the network, denoted as C₀, and the slot count for astation to start a new round of backing off, denoted as C₁. The slotcount is set to C₀ immediately after the association procedure, workingas a starting point for a station to access the medium. C₀ will not beused thereafter by the station. The value of C₀ should be chosencarefully to avoid overlapping slot counts among stations. The parameterC₁ is used to reset the slot count for round robin service throughout astation's lifetime in a network. All stations in the same network sharethe same value of C₁. Both the C₀ and C₁ should be determined during theassociation procedure of a new station, and C₁ should be adjusted as thenetwork evolves.

The AP and stations play different roles in the deterministic backoffprocedure of the present invention. The AP assumes the responsibilityfor selecting appropriate value for C₀ and C₁, and for distributing itschoice to the network. Whereas, each station is an executor performingthe deterministic backoff procedure of the present invention usingparameter settings selected by the AP. Hence, the AP works as ascheduler and coordinator during the deterministic backoff procedure ofthe present invention and all stations operate then as dispatchers. Itshould also be able to resolve network collisions by negotiating withthe stations to adjust their slot counts.

In the method of the present invention, the value of C_(l) is set to thenumber of stations (N) existing in the network, including the newlyassociated station and the AP.

C₁=N  (1)

The value of C₀ is set to a value that is not currently used by anystation in the network. The method of the present invention ensures thatthe collection of all the slot counts in use in the network should be aset with its member counting from 1 to the total number of stations. Inthis case, before a notification message of a successful association(such as an association response with status successful, or a newmanagement frame as shown in FIG. 3) is announced, all other stationsstill maintain the number of stations as (N−1), and its slot countshould be an integer between 1 and (N−1). Thus a reasonable choice ofthe initial slot count C₀ for the newly joined station is using theupdated station number N, which avoids overlapping with the currentlyused slot count and still preserves the continuity for the allocatedslot counts. That is,

C₀=N  (2)

where N is the number of all the stations (AP and clients), includingthe client which was newly associated to the network. Equations (1) and(2) are aimed at setting the initial counts for a station that newlyjoins the network. Obviously both parameters (C₀ and C₁) can be set bydelivering just one value: the number of the stations in the network. Amessage containing the number of stations can be piggybacked with theassociation response or be carried as a field in a management frameshown in FIG. 3.

Although in equation (1) the value of C₁ is set to the number ofstations in the network, it still can be set to any other value higherthan the number of stations in the network, which may result in lightermanagement overhead. In that case, the average backing off timeincreases, in an approximately linear relationship with C₁. However, inan alternative embodiment of the present invention, this may be employedto provide prioritized services among stations. For example, a simplepriority scheme could be implemented wherein a station with a higherpriority could be inserted multiple times in an address queue greaterthan N. For example, one station may have priority level three, meaningthat the station address could be inserted into the address queue threetimes. Thus, that station would have three chances to transmit duringany round of backoff periods. Another station may have priority leveltwo, meaning that the station address could be inserted into the addressqueue two times. Thus, that station would have two chances to transmitduring any round of backoff periods. While this embodiment reducessomewhat the overall fairness, it may be necessary or advantageousdepending on the type of traffic/data or the criticality of thetraffic/data being transmitted.

Note that in some situations where the number of stations in the networkis large but the only a small portion of them have data to send, it maynot be an efficient approach to set the value of C₀ and C₁ equal to thenumber of stations as in equations (1) and (2). For example, consider awireless local area network with one AP and as many as 30 associatedmobile stations. It is probably that the AP is the only sender in thenetwork most of the time. In this case, each time the AP has to wait fora period of 30 consecutive time slots before it can access the channel,which degrades the AP's and the network's performance compared to theconventional exponential random backoff method. Therefore, in analternative embodiment, the values of C₀ and C_(l) may be adjusted to anumber smaller than the number of stations in the network. This can beachieved by letting more than one station with little uplink trafficshare the same slot count. Each time the transmission opportunity comes(that is, slot count comes down to zero), each of these stations cantake actions complying with either of the following rules:

-   -   1) A station can transmit its frame with a predefined        probability of collision p.    -   2) A station can adopt a mechanism similar to the conventional        exponential random backoff method to determine whether to        transmit at this opportunity. That is, the station initiates a        counter and decrements the counter upon each transmission        opportunity belonging to itself. If the counter reaches zero,        the station decides to transmit, otherwise that station forgoes        the transmission opportunity.    -   3) Stations sharing a slot may be given the opportunity        deterministically to initiate a frames transmission once every        1/N_(ss) slots, where N_(ss) is the number of station sharing a        given slot. This would theoretically result in no collisions but        delay transmission opportunities for those stations sharing        slots.

In this embodiment with rules 1 and 2 above, there is a probability ofcollisions among the stations that share the same slot count. But theadvantage is that this alternative embodiment may increase the channelutility if appropriately used. Note that this alternative embodimentrequires changes in the address queue. Moreover, the AP shouldexplicitly inform those stations that are sharing slot count with othersby transporting a management frame to them or piggybacking theinformation onto data or control frames.

In the deterministic backoff method of the present invention, a stationhas to register itself with the AP and obtain the values for C₀ and C₁prior to starting the deterministic backoff procedure. In other words, astation cannot use the deterministic backoff method of the presentinvention to access the medium before there is a successful association.In fact, an inappropriate method employed by an unassociated mobilestation to access the medium, such as conventional random backoff, maycorrupt the ongoing deterministic backoff procedure, because theunintended chosen slot count may overlap with others and consequentlycause collisions.

This challenge is overcome by letting an unassociated station choose astatic deferring time, DIFS, to access the medium during the joiningprocedure. Note that equation (2) guarantees that, those associatedstations have to wait for at least a period/interval of DIFS plus oneslot time before gaining control of the medium. Thus, the setting ofdeferring a DIFS time shorter than that used by these associatedstations ensures that the transmission by an unassociated mobile stationwill not cause collisions within the ongoing backoff procedure.

However, a new issue arises if multiple unassociated stations access themedium simultaneously, which results in collisions among them. To solvethis problem, before each attempt to transmit the join requests, eachunassociated station is permitted to choose a random value from aninterval [0, JoinTimeOut] as the time for backing off. Once such aperiod has elapsed, the station can attempt to access the medium afterthe medium has been idle for a DIFS time. In actual practice, thepossibility of two or more stations joining the network simultaneouslyis fairly low. Most stations can successfully access the medium in theirfirst attempt.

It should be noted, that the PIFS (point inter-frame space) interface isnot used here, because it is reserved for the AP to conduct pointcoordination function (PCF)/hybrid coordination function (HCF)controlled channel access (HCCA) operations or for other emergencycases.

The issue of adjusting the slot count arises with the dynamics of thewireless networks. During the lifetime of a wireless network, somemobile stations may join the network at one time, and some may leave atanother time. In this situation, the number of associated mobilestations varies with time. In accordance with the principles of thepresent invention, the (re)setting of the backoff slot counter builds onthe global information—the number of stations—of the network so it isnecessary to adjust the setting adaptively according to the networkdynamics.

Each station should maintain information necessary for adjusting theslot count during the dynamic evolution of the wireless network. First,a variable (N_(i)) in each station holds a number representing thenumber of stations in the network. Its value is used to derive the valueof C₀/C₁ using equation (1) and (2). Each station updates the value ofthis variable when a station joins or leaves the network, by sniffingthe association/disassociation messages or by receiving a managementframe broadcast by the AP with its format shown in FIG. 3. Once updatedall stations should hold the same value of N₁. Second, a station shouldmaintain an address sequence that records the order of the round robinservice. Each address corresponds to one station. And the relativeposition of addresses in the sequence indicates the serving order forthese stations. Stations whose addresses are neighboring in the sequenceshould be served successively with a time interval/period of one DIFSplus one slot time. This sequencing service mechanism is achieved bysetting these slot counts in compliance with the address sequence. In analternative embodiment described above that includes a simple priorityscheme, N_(i) would be greater than the number of stations and somestations (with higher priority) would appear in the address queue morethan once. In a priority scheme each station may be assigned a priority(station priority scheme) or the priority could be assigned based on thepriority of the data/traffic that a station needs or wants to transmitso a station's priority dynamically changes over time (traffic priorityscheme). In the traffic priority scheme, the address queue would changeeach time the traffic priority changed.

Each station, including the AP, shares these two pieces of information(the number of stations and the address sequence of the stations) duringthe life cycle of the network. Many approaches can be employed toachieve this goal. For example, they can be collected by constantlymonitoring the wireless medium and the network activities of othermobile stations. Such a passive eavesdropping technique is easy andsimple but lacks reliability. In accordance with the principles of thepresent invention, the AP provides these pieces of information directlyto the network through broadcasting such information in managementframe. Each time an event, such as a mobile station joining or leavingthe network occurs, the AP transmits an additional frame containing theneeded information in a Short inter-frame spade (SIFS) timeperiod/interval after the (dis)association exchange. That is, this newframe can be viewed as the last frame within the (dis)associationexchange sequence.

Such an exemplary management frame is shown in FIG. 3 where the framebody includes the number of stations and the address sequence. It shouldbe remembered that an alternative embodiment having a simple priorityscheme would have a number of stations (N_(i)) greater than the actualnumber of stations and the address sequence may include the samestation(s) multiple times.

Thus, the (dis)association procedure can be described as follows.Whenever the AP receives a (dis)association request frame, the AP firstresponds with a (dis)association frame. Then a SIFS time period/intervallater after receiving an ACK, the AP broadcasts a new management frameto the network, carrying the following information: 1) the number ofstations in the network and 2) the address sequence. Mobile stationsreceiving this frame should update their stored information.

It is possible that this announcement message may not be correctlyreceived by some stations, which could lead to disagreement ofmaintained information among stations. In this case, the AP can 1)explicitly inform each station of the information by initiating reliableunicast session or 2) periodically announce the information in thebeacon messages. Other approaches that provide reliable delivery ofinformation can also be applied.

The address sequence can serve as a basis for a mobile station tocalibrate the slot count, given that it can capture the frames in theair. As an instance, when station j has concluded its transmission andat least one frame of this transaction has been captured by station i,then station i can use the address sequence to recalculate its slotcount slot(i) using the following equation:

slot(i)=(seq(i)−seq(j))mod(C₁)  (3)

where seq(i) and seq(i) denote the relative position (named sequencenumber) of the addresses of station i and j in the address sequencerespectively. Since the AP is always present in the network, its addressis first and it gets address sequence number 1.

Using equation (3), a station can calibrate its slot count at eachconclusion of a frame exchange. Intrinsic characteristics of a sharedwireless medium facilitate the calibration procedure, as a frame in theair can be sensed by all interfaces sharing the medium. Moreover, thecalibration procedure can be used to resolve collisions that are causedby controlled channel access (CCA) error or obsolete NAV (networkallocation vector) updates. For example, if the data frames of twomobile stations collide at one slot time, then before the next round ofservice for them, both stations can recalculate their slot count usingthe calibration procedure to avoid further collisions.

Each station decrements its slot count after the medium has been sensedand determined to be idle for a DIFS time period/interval, even in thecase that it does not intend sending any data. The slot count cyclesthrough C₁ to 0 ceaselessly for a station's lifetime. Every time the sotcount reaches zero, the station initiates a new frame exchange sequence,or does nothing, both followed by a new round of slot counting. Thismechanism is introduced to preserve the mutual relationship betweenthese distributed slot counts within the network's operating procedure.

FIGS. 4 a and 4 b are flowcharts showing the operation of thedeterministic backoff method of the present invention. An unassociatedstation should first perform an association procedure to join thenetwork using the reserved DIFS time interval. The AP informs the newstation of its use of a deterministic backoff method and of the initialslot count during this exchange. At this point, the station determinesif it wishes to become a member of the network considering the backoffmethod. The station may be a legacy station that is unable to supportthe deterministic backoff method of the present invention. Once thestation becomes a member of the network (joins the network or becomesassociated with the network), the station cycles its slot count throughN to 0, where N is at least the number of stations in the network.Normally, N is the number of stations in the network but as describedabove, N may advantageously be greater than the number of stations inthe network for alternative embodiments described above. Each time theslot count reaches 0, the station gets an opportunity to initiate aframe transmission. Moreover, the station adjusts its slot count afterwaiting a predetermined period of time and based on information of theserving order among stations throughout the lifetime of the stationwithin the network.

FIG. 4 a is a flowchart of the operation of the deterministic backoffmethod of the present invention form the perspective of the accesspoint. At 405 the AP receives a request from a station to become amember of (join or become associated with) the network. The AP checks at410 to determine if the station is already associated with (a member of)the network. If the joining station is not already a member of thenetwork then at 420 the AP sends the joining station an initial slotcount, the communications medium access method used by the network, thenumber of stations in the network and the address queue. The AP thenwaits to receive either an acknowledgment from the joining station or adissociation message from the station. A dissociation message would besent, for example, if the joining station was a legacy station thatcould not support the communication medium access method of the presentinvention. If the joining station is already associated with the networkthen the AP waits a predetermined time period at 450. The slot count isadjusted at 425. In an exemplary embodiment the slot count isdecremented by one. An adjustment that is incremented could also beused. The slot count is compared to a predetermined value at 430. Theexemplary embodiment of FIG. 4A compares the adjusted (decremented) slotcount to 0. If the slot count has reached the predetermined value thenat 435 the AP determines if the AP has a data frame to transmit. If theAP has a data frame to transmit then data frame transmission isinitiated at 440. If the AP does not have a data frame to transmit thenthe AP skips its turn and selects a new slot count at 445. Once dataframe transmission has been initiated then the AP selects a new slotcount at 445. If the slot count had not reached the predetermined valueat 430, then the AP waits a predetermined time period at 450. Of course,if the AP has not received a new request to join the network then 405,410, 415, 420 and 450 are skipped/not executed.

FIG. 4 b is a flowchart of the operation of the deterministic backoffmethod of the present invention form the perspective of the station. At460 the joining station sends a request to join the network to the AP.The joining station then waits until it receives the communicationsmedium access (deterministic backoff) method from the AP, as well as theslot count, the number of stations in the network and the address queueat 465. At 470, the joining station determines if it supports thecommunication medium access (deterministic backoff) method used by thenetwork. If the joining station determines that it supports thecommunication medium access method then it saves the slot count, theaddress queue and the number of stations in the network at 480. Thestation proceeds as described above to gain access to the communicationmedium in accordance with the principles of the present invention. Ifthe joining station determines that is does not support thecommunication medium access method of the present invention then itsends the PA a dissociation message at 475. Of course, if the joiningstation is already a member of (associated with) the network then 460,465, 470, 475 and 480 are skipped/not executed.

FIG. 5 a is a schematic/block diagram of the operation of an AP ingaining access to a communication medium in a contention-based networkin accordance with the principles of the present invention. Associationmodule 505 receives any requests to join and any acknowledgments ordissociation messages from stations. The association module performs anyoperations relating to a station joining (becoming a member of) thenetwork and sends out any messages relating to a station joining thenetwork via transmission module 510, which handles transmission of anymessages associated with a station joining the network as well as anydata the AP has to transmit once the AP gains access to thecommunication medium. The transmission module also handles any encoding,encryption and modulation. The data module 515 handles preparing anydata that the AP wants/needs to transmit via the transmission module.The above description is of an exemplary embodiment and the modules mayin fact, be combined into a single module or further subdivided intoadditional modules such as an encoder, encrypter, modulator.

FIG. 5 b is a schematic/block diagram of the operation of a station ingaining access to a communication medium in a contention-based networkin accordance with the principles of the present invention. Associationmodule 520 sends a request to join the network and receives anindication of the communication medium access method, the slot count,the number of stations in the network and the address queue from the AP.The association module further determines if the joining stationsupports the communication access method then it sends the AP anacknowledgment message. If the joining station does not support thecommunication access method then it sends the AP a dissociation messagein order to dis-associate from the network. That is, the associationmodule performs any operations relating to a station joining (becoming amember of) the network and sends out any messages relating to a stationjoining the network via transmission module 525, which handlestransmission of any messages associated with a station joining thenetwork as well as any data the station has to transmit once the stationgains access to the communication medium. The transmission module alsohandles any encoding, encryption and modulation. The data module 515handles preparing any data that the AP wants/needs to transmit via thetransmission module. The above description is of an exemplary embodimentand the modules may in fact, be combined into a single module or furthersubdivided into additional modules such as an encoder, encrypter,modulator.

It should be noted that the receipt of data is not shown or describedbecause it is unaffected by the method of the present invention. Datais, of course, received and processed by the AP and stations that aremembers of the network but the present invention is directed to a methodand apparatus for gaining access to a communication medium in acontention-based network in order to initiate frame transmission.

The deterministic backoff method of the present invention handles theissue of medium access in a totally different manner than traditionalCSMA/CA approaches. In CSMA/CA, a random backoff method is used to avoidcollisions, while the method of the present invention reduces oreliminates collisions by deterministically assigning a value to the slotcounter. The two solutions are incompatible. A station using a randombackoff method cannot work within a network using deterministic backoffas its medium access method.

The AP informs the potential new joiner which algorithm the network usesfor medium access. A legacy station that only supports a random backoffmethod should withdraw its association request once it determines thatthe network is running a deterministic backoff method. Only requestsfrom those stations that support the deterministic backoff method shouldbe accepted by the AP in this situation.

The relationship between the two algorithms is similar to that of theTDMA (time division multiple access) and CSMA (carrier sense multipleaccess): they are exclusive and only one of them can exist within anetwork. However, it should be noted that the method of the presentinvention still falls into the category of CSMA, as 1) its operationsare based on the carrier sensing mechanism provided by the low-levelphysical layer, and 2) the AP does not control time scheduling as inTDMA. In fact, there is no time scheduling in method of the presentinvention rather transmission opportunities are gained by stations in around robin manner.

The round robin service provided by the deterministic backoff methodguarantees the fairness for medium access among stations. Each stationhas an opportunity to complete a frame transaction in each serviceround. Moreover, if a time limit is imposed on the time length for eachtransmission opportunity, denoted as TxopLimit, then the serviceinterval can be bounded to ((m−1)·(TxopLimit+DIFS)+DIFS), with m beingthe number of stations in the network. Thus, with carefully chosenTxopLimit and appropriate control of the network's size, it is possibleto provide guaranteed QoS for network applications.

In an alternative embodiment already described above, a simple priorityscheme can be implemented by increasing the number of stations andinserting stations with higher priority traffic/data into the addressqueue multiple times. This, of course, impacts the overall fairness ofthe deterministic backoff method of the present invention but may beadvantageous in some cases.

Network collisions are an annoying issue for the CSMA based wirelesscommunications, as collisions greatly degenerate network performance,particularly in terms of throughput and network efficiency. However,collisions are eliminated (or greatly reduced) in the deterministicbackoff method of the present invention. Each station can exclusivelytake control of the wireless medium after its slot count reaches zero.In this sense, the deterministic backoff method of the present inventionoutperforms legacy random backoff methods.

Another advantage of the deterministic backoff method of the presentinvention is that the network efficiency is extremely high assuming thateach station has pending data to send when it gets control of thechannel. In this situation, the time interval between two successivetransmission opportunities is only one DIFS time period/interval plusone slot time, about 70 βs for IEEE 802.11b and an even shorter time forIEEE 802.11a and IEEE 802.11g. Thus, the ratio of the idle period to thebusy period is low, and the network efficiency is high. In such asaturated network, the network behavior is similar to that of TDMAnetworks.

It is to be understood that the present invention may be implemented invarious forms of hardware (e.g. ASIC chip), software, firmware, specialpurpose processors, or a combination thereof, for example, within aserver, an intermediate device (such as a wireless access point or awireless router) or mobile device. Preferably, the present invention isimplemented as a combination of hardware and software. Moreover, thesoftware is preferably implemented as an application program tangiblyembodied on a program storage device. The application program may beuploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof), which is executed viathe operating system. In addition, various other peripheral devices maybe connected to the computer platform such as an additional data storagedevice and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying figuresare preferably implemented in software, the actual connections betweenthe system components (or the process steps) may differ depending uponthe manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the related art will be ableto contemplate these and similar implementation or configurations of thepresent invention.

1. A method for gaining access to a communication medium in acontention-based network, said method comprising: determining a slotcount based on a number of stations in said contention-based network;adjusting said slot count; initiating a frame transmission when saidslot count reaches a predetermined value; and calibrating said slotcount upon completion of said frame transmission.
 2. The methodaccording to claim 1, wherein said stations in said contention-basednetwork share said communication medium.
 3. The method according toclaim 1, further comprising receiving a request to join saidcontention-based network.
 4. The method according to claim 3, furthercomprising sending information regarding the communication medium accessmethod used by said contention-based network, said number of stations,said slot count and an address queue.
 5. The method according to claim4, further comprising receiving an acknowledgment.
 6. The methodaccording to claim 4, further comprising receiving a dissociationmessage.
 7. (canceled)
 8. A method for gaining access to a communicationmedium in a contention-based network, said method comprising: receivinga slot count based on a number of stations in said contention-basednetwork; adjusting said slot count; initiating a frame transmission whensaid slot count reaches a predetermined value; and calibrating said slotcount upon completion of said frame transmission.
 9. The methodaccording to claim 8, wherein said stations in said contention-basednetwork share said communication medium.
 10. The method according toclaim 8, further comprising sending a request to join saidcontention-based network.
 11. The method according to claim 10, furthercomprising receiving information regarding the communication mediumaccess method used by said contention-based network, said number ofstations, said slot count and an address queue.
 12. The method accordingto claim 11, further comprising sending an acknowledgment.
 13. Themethod according to claim 11, further comprising sending a dissociationmessage. 14-28. (canceled)